Pressure relief devices for use with fluids held under pressure in various pressure vessels are known. Typically, a pressure relief device includes a plug positioned in a passage within a housing attached to a pressure vessel, with the plug normally in a closed position. An inlet of the passage is in communication with the fluid in the pressure vessel when the plug is closed. In a conventional pressure relief device, the plug blocks the flow of the fluid through the passage when the plug is in the closed position, and the plug is movable to an open position in which the fluid can exit the pressure vessel via the passage.
The conventional pressure relief device is intended to relieve excess pressure to which the fluid in a pressure vessel is subjected or, alternatively, to release the fluid in the event of fire. Such excess pressure may be caused, for example, by a fire. However, fire does not always result in excess pressure. For example, where a pressure vessel is holding fluid at only one-half maximum nominal pressure, a fire may not raise the pressure beyond the maximum nominal pressure. In these circumstances, however, the fire would weaken the tank wall, resulting in a dangerous situation. Known pressure relief devices are designed to function in the event that a fire breaks out at or near the pressure vessel, resulting in a risk of explosion or collapse of the pressure vessel.
It is important that the pressure relief device opens quickly when conditions require. In a typical temperature-controlled pressure relief device, the plug is maintained in the closed position by a fusible alloy with a relatively low melting point. The fusible alloy melts when the fusible alloy is exposed to temperatures equal to or greater than its melting point, for example, upon a fire breaking out in the vicinity of the pressure vessel. After the fusible alloy has melted, the plug can be moved to the open position.
In many known pressure relief devices, a spring is used to urge the plug to the open position. The spring is intended to cause the plug to move quickly from the closed position to the open position when the fusible alloy has melted. Also, the spring has the additional purpose of ensuring that the plug moves completely to the open position, and does not become lodged at a partly open position in which the passage is partially blocked. Partial blockage is undesirable because it would impede the flow of fluid through the passage. An example of a spring being used to urge a plug to an open position is disclosed in U.S. Pat. No. 2,194,541 (Buttner) (see FIGS. 1 and 3 therein).
However, known pressure relief devices suffer from a number of defects. For example, because the pressure relief device typically remains in the closed position for an extended period of time, the fusible alloy may creep (i.e., deform over time under stress) because of the stress imposed on the fusible alloy by the pressure force from the fluid and/or the spring. Ultimately, creep can result in the plug moving to the open position (or a partly open position). and consequent release of the fluid, in circumstances where release of the fluid is neither necessary nor desirable.
Another defect of conventional pressure relief devices is that, in some devices, the movement of the plug to the open position results in the expulsion of the plug from the pressure relief device at a relatively high velocity. The sudden expulsion of the plug can be dangerous and damaging.
Also, because relatively rapid flow of the fluid through the passage upon the plug moving to the open position is necessary, the path of the flow of the fluid through the passage should be relatively unobstructed. However, conventional pressure relief devices typically include a number of sharp edges or sharp corners in the passage which is opened up when the plug moves to the open position. The sharp edges or sharp corners should be minimized, however, in order to maximize flow when the plug is in the open position.
In addition, known pressure relief devices also tend to have a relatively large number of mechanical parts, so that the risk of failure or malfunctioning of these devices is relatively high. For example, in many known pressure relief devices, a seal member is required in order for the plug to block flow completely when the plug is in the closed position. However, when the plug moves to the open position, the seal member is in the path of the fluid, and the seal member therefore tends to obstruct the fluid flow through the passage in the pressure relief device.
There is therefore a need for an improved pressure relief device.